Ardipithecus: Foot

I’ve fallen a bit behind in posting about the Ardipithecus remains. Here are some things about the foot, which is quite an interesting piece of the puzzle.

What they have of the foot is nothing too shabby: a talus, medial and intermediate cuneiforms, a cuboid, the first three metatarsals (MT1-3), and some phalanges.

I mentioned a few weeks ago that one of the most striking features of the foot is the abducted hallux–Ardi had a grasping big toe, like apes. They can tell this from the orientation of the medial cuneiform’s articular facet for the MT1. In addition, the joint surfaces of the two bones show that the MT1 could rotate about the joint. In humans and other bipedal hominins, these surfaces are more or less flat. Ardi’s foot is a big deal because what we have of hominin feet up to this point suggest a more-or-less human-like ability to transmit forces from bipedal walking, and MT1 plays a major role in this (that’s why our big toes are so big and stupid looking).

But Ardi couldn’t have walked bipedally this way. Rather, the authors posit that walking-forces were largely transmited through MT2-3. They point out some interesting features to support this: relative to their length, the bases of these bones are fairly tall; the sphericity of their heads (especially on the superior surface) hint that the toes were hyperextended. Additionally, notches on the dorsal aspects of both the MT2 base and the intermediate cuneiform (which articulates with the MT2) were probably caused by habitual pressure caused by the tarso-metatarsal joint capsule, possibly from “upright walking and running” (Lovejoy et al. 2009, p. 72e9). Well, I don’t know about running. One thing I noticed about the MT3 head is that, while it is fairly “domed” as in humans, with a transverse depression just behind the dorsal surface as in humans, it also has a large tuberosity behind the depression, which looks fairly similar when viewed from the side (but not totally) to the transverse ridges on African ape metacarpals. In apes, these ridges prevent the fingers from hyperextending–this similar-looking feature in Ardi’s foot could also have prevented hyperextension of the toe (?), not very biped-like. Of course, who says there’s only one way to be bipedal?

The talus–the bone sitting the center of your ankle–is variously ape- or monkey-like in how the joint contacts the tibia, suggesting the tibia was fairly obliquely oriented on the joint (in us bipeds it’s more or less perpendicular) (DeSilva 2009). Ardi’s cuboid also gets a lot of air-time in the paper. The cuboid is a little wedged bone sitting on the side of your foot, and because of this lateral position, the tendon of the fibularis longus muscle crosses over the side and inserts into the base of the MT1 (and of the medial cuneiform in humans). In humans and Old World monkeys, this surface also bears an articular surface for the ‘os peroneum,’ a small bone sitting within the tendon. Apparently this articular surface is lacking in apes. For Ardi, the authors discover a fairly monkey-like morphology for this surface. Ardi’s cuboid, then, indicates a monkey-like ability to adduct the big toe, but also support the structural integrity of the middle foot.

In sum, like in the pelvis paper, the authors posit more bipedal functionality than I’d be comfortable making. The foot of Ardipithecus ramidus was certainly something interesting, capable of ape-like grasping, and probably a non-trivial amount of dorsiflexion at the ankle, as well (read: ‘capable climber’). At the same time, it is not clear to what extent it was used for ape-like climbing and/or monkey-like quadrupedalism. And if it was walking bipedally, it was doing it quite differently from any human or other fossil hominin. Hey, maybe this is the foot you need to be bipedal in the trees?

Just one more thing: I’m not terribly familiar with what feet are available from Miocene apes–a decent outgroup for comparing the functional/phylogenetic morphology of fossil feet. But, again like in the pelvis paper, this paper doesn’t mention Oreopithecus, which may be a good comparison to make. Oreopithecus was an Italian insular ape some 7 or so million years ago. It has also been argued, fairly recently, to be bipedal based on a lordotic lumbar spine, presence on the innominate of an anterior inferior iliac spine, and the architecture of the internal trabecular bone of the ilium (Rook et al. 1999; Kohler and Moya-Sola 1997). For the longest time, the enigmatic morphological convergences between Oreopithecus and hominins have been attributed to the former’s unique insular habitat: absence of predators in this unique enviroment allowed for upright posture, and possibly even bipedalism, to evolve in this now-extinct ape. But seeing what all this shares with Ardipithecus–hominin or not–it may be that such upright, “arboreal bipedal” positional behavior is the ancestral hominoid condition.

ReferencesDeSilva J. 2009. Functional morphology of the ankle and the likelihood of climbing in early hominins. Proceedings of the National Academy of Sciences 106: 6567-6572.

Kohler M and Moya-Sola S. 1997. Ape-like or hominid-like? The positional behavior of Oreopithecus bambolii reconsidered. Proceedings of the National Academy of Sciences 94: 11747-11751.